Much cargo is presently carried in cargo containers. Such cargo containers can be quite large (from 10 feet to 50 feet in length or more). When such containers are unloaded from ships, rail cars or aircraft, at ports of entry into a country, they should in theory be inspected by customs authorities. However, because the volume of such container traffic is high, and because each container may be loaded with sealed boxes, drums and the like, it is impossible in practice to inspect the contents of each container. In fact, because of the high volume of such traffic, there may be only a few minutes available to inspect each container.
X-ray examination of such containers has been proposed. Such x-ray examination is capable of detecting certain types of contraband, such as firearms, but it is not capable of detecting other kinds of contraband, such as drugs, and explosives if such substances are appropriately packaged.
Attempts have therefore been made to detect contraband such as drugs, firearms and explosives by other means, such as by detecting vapors emitted by these items. However, these methods have resulted in high false alarm rates and unreliable detection of the substances, for various reasons. Among the reasons are the extremely low vapor pressures of many of the substances which are to be detected. In addition, although many contraband substances emit vapors from their breakdown products or from products associated with their manufacture, such vapors may also be emitted by other industrial products, resulting in false alarms. The unnecessary inspections resulting from the false alarms can, due to their frequency, cause a breakdown of the entire detection process.